Implantable stimulators are used to treat a variety of patient medical conditions. Such stimulators include a microminiature implantable electrical stimulator, referred to herein as a microstimulator, and known as the BION® microstimulator. The BION microstimulator has been developed (by Advanced Bionics of Valencia, Calif.) to overcome some of the disadvantages of traditional leaded systems. The standard BION device is a leadless microstimulator, as the implantable pulse generator and the electrodes have been combined into a single microminiature package. A standard configuration of the BION device is a cylinder that is about 3 mm in diameter and between about 2 and 3 cm in length. This form factor allows the BION device to be implanted with relative ease and rapidity, e.g., via endoscopic or laparoscopic techniques. With this configuration, the BION device consists of only two electrodes: a reference, or indifferent, electrode at one end and an active electrode at the other end. In addition, with this configuration, electrical signals delivered to nerves travel away from the stimulation location along the nerve fibers in both directions.
The teachings of the present disclosure provide a means of chronically stimulating the nerves and muscles that contribute to the function of the gastrointestinal system with the BION device. Electrical stimulation of such targets may provide significant therapeutic benefit in the management of gastrointestinal motility disorders, sphincteric disorders, and obesity.
Gastrointestinal disorders may be due to a number of underlying etiologies and may manifest a number of symptoms. Gastrointestinal motility disorders, sphincteric disorders, and eating disorders may be caused by musculature that fails to contract, musculature that fails to relax, or musculature that fails to contract and relax in a coordinated fashion. An understanding of these various diseases requires some description of the anatomy and the physiology of the gastrointestinal system.
The gastrointestinal (GI) system consists of the gastrointestinal tract and associated glandular organs that produce secretions. The major structures of the gastrointestinal tract are the mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, and ileum), large intestine (colon), rectum, and anus. Associated glandular organs include the salivary glands, liver, gallbladder, and pancreas.
Most of the GI tract is covered with a thin outermost layer known as the serosa (adventitia), which is primarily connective tissue. Just underneath the serosa is the muscularis externa, which consists of two substantial layers of smooth muscle cells: an inner circular layer and an outer longitudinal layer. Contractions of the muscularis externa mix and circulate the contents of the GI lumen and propel the contents along the GI tract. The layer just below the muscularis externa is the submucosa, which is primarily connective tissue and may also contain secretory glands in some portions. The innermost layer of the GI tract is the mucosa, which is rich in glands, blood vessels, and lymph tissue.
The wall of the GI tract contains many interconnected neurons. The most prominent plexus is the myenteric plexus (Auerbach's plexus), which is located between the outer longitudinal and the inner circular layers of the muscularis externa. The submucosal also contains a major plexus, the submucosal plexus (Meissner's plexus). These two plexuses, along with the other neurons of the GI tract, constitute the enteric nervous system. The enteric nervous system includes both sensory and motor fibers, and many of these are connected to form reflex arcs that can coordinate activity in the absence of extrinsic innervation. Approximately 108 neurons reside in the GI tract, a number comparable to that of the spinal cord.
The sympathetic nervous system also contributes to the innervation of the GI tract, including fibers stemming from the prevertebral and paravertebral ganglia and coursing through the celiac, superior mesenteric, inferior mesenteric, and hypogastric plexuses. Activation of the sympathetic nerves usually inhibits the motor and secretory activities of the GI system. Most of the sympathetic fibers terminate on neurons in the myenteric and submucosal plexuses and not directly on smooth muscle or glands. The sympathetic nervous system also appears to induce contraction of some sphincters.
Parasympathetic innervation of the GI tract arises primarily from the vagus nerve, but the anus, rectum, and distal part of the large intestine receive parasympathetic fibers from the pelvic nerves. The parasympathetic fibers are typically preganglionic, and they synapse with the postganglionic fibers in the wall of the GI tract. These postganglionic fibers directly innervate the smooth muscle and secretory cells of the GI tract.
In GI smooth muscle, the resting membrane potential characteristically varies or oscillates. These baseline oscillations are referred to as slow waves, and the frequency varies from about 3 per minute in the stomach to about 12 per minute in the duodenum. Chemoreceptors and mechanoreceptors, as well as parasympathetic and sympathetic innervation, may modulate these slow waves or may lead to independent and increased activity in response to a food bolus.
One of the primary functions of the GI tract is peristalsis, which is the progressive contraction of successive sections of the inner circular smooth muscle layer of the muscularis externa. These contractions move along the GI tract in order to propel food from the esophagus to the anus. In much of the GI tract, a bolus of food that produces mechanical distention of the GI tract will typically cause contraction of the circular smooth muscle immediately behind the bolus and relaxation of the circular smooth muscle immediately in front of the bolus. The slow waves of the smooth muscle cells tends to determine the timing of peristaltic contractions.
The esophagus is responsible for the transit of food from the pharynx to the stomach. Interestingly, the muscle tissue of the upper one-third of the esophagus is striated, while the muscle tissue of the lower one-third of the esophagus is smooth. Neurons of the myenteric plexus directly innervate the smooth muscle cells of the esophagus and communicate extensively. The esophageal musculature, both striated and smooth, is extrinsically innervated primarily by branches of the vagus nerve. The upper esophageal sphincter (UES) prevents the entry of air into the esophagus. The lower esophageal sphincter (LES) prevents the entry of gastric contents into the esophagus. Normally, when a wave of esophageal peristalsis begins, a reflex causes the LES to relax. Esophageal motility disorders include lack of peristalsis and diffuse esophageal spasm, a medical condition in which the lower part of the esophagus contracts in a prolonged and painful fashion after swallowing. Some individuals suffer from dysfunction of the LES, which may allow gastric juice to move back into the esophagus and erode the esophageal mucosa. This is known as gastroesophageal reflux disease (GERD). Chronic erosion by GERD can lead to a precancerous medical condition known as Barrett's esophagus. Other individuals suffer from achalasia, in which the LES fails to relax sufficiently during swallowing to allow food to enter the stomach.
The major functions of the stomach are to serve as a reservoir following a meal, to break food into smaller particles and begin digestion, and to empty gastric contents into the small intestine at a controlled rate. Normally, when a wave of esophageal peristalsis begins, a reflex causes the upper (proximal) portion of the stomach to relax. When food enters the stomach, gastric contractions begin, which are usually very strong slow wave contractions at the rate of about 3 per minute. These contractions begin near the upper (proximal) portion of the stomach and travel down towards the gastroduodenal junction, also known as the pylorus. As each peristaltic wave reaches the pylorus, the pyloric sphincter snaps shut, so that the stomach empties in small squirts, one for each peristaltic wave. The rapid contractions also serve to break up large food particles and to mix food with digestive juices. If gastric emptying occurs too rapidly, a duodenal ulcer may develop. If the pyloric sphincter allows regurgitation of the duodenal contents, then a gastric ulcer may develop. Some patients suffer from gastroparesis, which is a decrease in or a lack of coordination of gastric contractions.
The small intestine makes up about three-fourths of the length of the GI tract. It is approximately 5 meters in length, and a bolus traverses the small intestine in 2 to 4 hours. The small intestine is the site where most digestion and absorption take place. In addition to peristalsis, the small intestine has a more frequent movement known as segmentation, which is characterized by closely spaced contractions of the inner circular muscle layer of the muscularis externa. These contractions divide the small intestine into small neighboring segments. In rhythmic segmentation, the sites of the circular contractions alternate, so that an individual segment of intestine contracts and then relaxes. Segmentation effectively mixes chyme (digested matter) with secretions and brings material into contact with the mucosal surface for absorption. At the distal end of the small intestine (i.e., the ileum) is the ileocecal sphincter (also known as the ileocecal valve). Normally this sphincter is closed; however, short-range peristalsis in the ileum relaxes the sphincter and allows a small amount of chyme to squirt into the cecum (the first part of the large intestine). Administration of codeine and other opiates decreases small intestinal motility. This increases transit time and allows for more thorough absorption of water, salts, and nutrients, thus markedly reducing the frequency and volume of stools. Patients who have undergone abdominal surgery may experience dysfunction in small or large intestinal motility.
The large intestine (colon) receives about 1 liter of chyme per day. Most of the salts and water entering the colon are absorbed; the feces normally contain only 50-100 ml of water per day. The colon differs from the rest of the GI tract in that the longitudinal muscle of the colon is concentrated in three bands known as the taeniae coli. Additionally, the “peristaltic” wave of the colon is referred to as mass movement. It differs from a normal peristaltic wave in that the contracted segments remain contracted for a significant period of time. Mass movements push the colonic contents a significant length towards the rectum. In Hirschsprung's disease, also known as congenital aganglionic megacolon, enteric neurons are congenitally absent from a part of the colon. Typically only the internal anal sphincter and a short length of adjacent colon are affected. Filling of the rectum by a mass movement leads to reflex relaxation of the distal rectum in a normal person. In patients with Hirschsprung's disease, this reflex relaxation does not occur, and as a result functional obstruction of the distal colon occurs. This leads to dilation of the colon (i.e., megacolon) above the obstruction.
The anal canal is usually kept closed by the internal and external sphincters. The internal anal sphincter is a thickening of the circular smooth muscle of the anal canal. The external anal sphincter is more distal and consists entirely of striated muscle. The external anal sphincter is innervated by somatic motor fibers via the pudendal nerves. This innervation allows the anal sphincter to be controlled both by reflexes and voluntarily. Patients with fecal incontinence are unable to regulate one or both of these sphincters.
In the fasting state, the slow wave activity of the GI tract is significantly attenuated. However, the GI tract continues to have periodic peristaltic contractions to propel food, especially large particles of food, towards the large intestine. These fasting contractions are known as the migrating myoelectric complex (MMC). The MMC begins in the stomach every 75 to 90 minutes in the fasting state, and it travels slowly all the way down to the small intestine.
Some of the GI disorders mentioned above may be treated with medication or with surgery. Some of these, such as fecal incontinence, may not respond well to treatment with either. An overview of three major GI disorders are as follows:
Gastroesophageal Reflux Disease (GERD)
Gastroesophageal reflux disease (GERD), which accounts for 75% of esophageal pathology [DeMeester, et al., “Gastroesophageal Reflux Disease.” In: Moody, et al, eds. Surgical Treatment of Digestive Disease. Chicago, Ill.: Year Book Medical Publishers; 1989: pages 65-108.], is characterized by a broad spectrum of clinical presentations, from simple heartburn to ulcerative esophagitis, esophageal stricture, and Barrett's metaplasia with its tendency to become malignant.
As many as 10 percent of Americans have episodes of heartburn (pyrosis) every day, and 44 percent have symptoms at least once a month [Orlando R. C., “The Pathogenesis of Gastroesophageal Reflux Disease: the Relationship Between Epithelial Defense, Dysmotility, and Acid Exposure.” Am J Gastroenterol 1997; 92(4 suppl): pages 3S-5S; and Isolauri, et al., “Natural Course of Gastroesophageal Reflux Disease: 17-22 Year Follow-up of 60 Patients.” Am J Gastroenterol 1997; 92: pages 3741.]. In all, GERD affects an estimated 25 to 35 percent of the U.S. population [Eisen, et al., “The Relationship Between Gastroesophageal Reflux Disease and Its Complications with Barrett's Esophagus.” Am J Gastroenterol 1997; 92: pages 27-31.]. Approximately one-third of the patients with heartburn who seek medical care have endoscopic evidence of esophagitis and about 10% to 20% have severe complications of esophagitis [Richter J. E., “Surgery for Reflux Disease—Reflections of a Gastroenterologist”, N Engl J Med. 1992; 326: pages 825-827. Editorial.].
The major complications of GERD are erosive/ulcerative esophagitis, esophageal stricture, and Barrett's esophagus (a precancerous medical condition), all of which result from the damage inflicted by gastric juice on the esophageal mucosa and changes caused by subsequent repair and fibrosis [Peters, et al., “Gastroesophageal Reflux.” Surg Clin North Am. 1993; Vol. 73: pages 1119-1144.]. An estimated 10% of patients treated for GERD have peptic stricture. A recent population-based study revealed an autopsy-estimated prevalence of Barrett's esophagus of 376 per 100,000, a greater than 16-fold increase from that of the 22.6 per 100,000 clinically diagnosed cases [Cameron, et al., “Prevalence of Columnar-Lined (Barrett's) Esophagus. Comparison of Population-Based Clinical and Autopsy Findings”, Gastroenterology. 1990; Vol. 99: pages 918-922.]. Patients with GERD are considered to have a greater incidence of Barrett's esophagus compared to others undergoing endoscopy [Green P H R. “What is the Prevalence of Barrett's in the General Population and in Patients with GERD?” Syllabus. NY Soc Gastrointest Endosc Postgrad Course. Nov. 1-11, 1994: pages 2-3.]. The development of Barrett's epithelium remains the most disconcerting complication of reflux disease because of its predisposition to carcinoma [Clouse, et al., “The Esophagus.” In: Gitnick G, ed. Current Gastroenterology. Volume 14. St. Louis, Mo.: Mosby-Year Book, Inc; 1994: pages 1-28.]. Two recent studies suggest the incidence of adenocarcinoma is one in 96 to 99 patient-years in follow-up [Williamson, et al. “Barrett's Esophagus: Prevalence and Incidence of Adenocarcinoma.” Arch Intern Med. 1991; Vol. 151: pages 2212-2216; and Miros, et al., “Only Patients with Dysplasia Progress to Adenocarcinoma in Barrett's Oesophagus.” Gut. 1991; 32: pages 1441-1446.].
Only 5 to 10 percent of patients with erosive esophagitis will fail to heal within 3 months on standard doses of Acid Pump Inhibitor medication (e.g., omeprazole, lansoprazole) [Bardhan K. D., “Is There Any Acid Peptic Disease that is Refractory to Proton Pump Inhibitors?” Aliment Pharmacol Ther, 1993; Vol. 7 (suppl 1): pages 13-31.]. However, esophagitis tends to become a relapsing, chronic medical condition. It recurs in 50 to 80 percent of affected patients within six to 12 months after the discontinuation of pharmacologic therapy [Fennerty, et al., “The Diagnosis and Treatment of Gastroesophageal Reflux Disease in a Managed Care Environment: Suggested Disease Management Guidelines.” Arch Intern Med 1996; Vol. 156: pages 477-84.]. Surgical antireflux procedures provides good to excellent relief of reflux symptoms in 84% to 89% of patients [Peters, 1993]. Such procedures involve reduction of a hiatal hernia, if present, as well as construction of a valve mechanism.
It is estimated that worldwide over $14 billion is spent each year for medications to relieve the symptoms of GERD. As medication only treats the symptoms but not the underlying cause of the disease, most GERD patients need to take daily doses of medication for a lifetime. For a small number of patients, a surgical procedure called fundoplication is available.
Achalasia
Achalasia is a disorder of the esophagus characterized by the reduced ability to move food down the esophagus and the inability of the lower esophageal sphincter to relax in response to swallowing. The disorder is characterized by loss of the wave-like contraction of smooth muscles that forces food through the digestive tract (peristalsis). The medical condition also includes spasms of the valve (sphincter) from the esophagus to the stomach that does not relax and lack of nervous stimulation of the esophagus. Causes include damage to the nerves to the esophagus, parasitic infection, and hereditary factors. Achalasia may occur at any age but increases with frequency with advancing age. The incidence is 2 out of 10,000 people.
The two most common symptoms of achalasia are dysphagia (inability to swallow) and regurgitation of food. Additional symptoms of achalasia may include chest pain that increases after eating, weight loss, drooling, and cough. Complications of achalasia include tearing (perforation) of the esophagus and gastroesophageal reflux disease (GERD).
The approach to treatment is to reduce the pressure at the lower esophageal sphincter (LES). This may be achieved by manipulating the lower esophagus sphincter by special instruments. Medications such as long-acting nitrates or calcium channel blockers may also be used to lower the pressure at the lower esophagus sphincter. Sometimes, botulinum toxin is injected to relax the sphincter. Surgery to decrease the pressure in the lower sphincter (called an esophagomyotomy) may be indicated if other interventions fail. The effectiveness of treatment varies from 60 to 85%, depending on the procedure.
Obesity
Obesity is characterized as an eating disorder causing excessive bodily fat which results from gastrointestinal disorders as described above, and emphasized herein as the inability of the sphincters muscles to contact and open in a normal coordinated fashion. Obesity affects millions of Americans, and a substantial percentage of these people are morbidly obese. These people may also suffer from such obesity-related problems as heart disease, vascular disease, and social isolation. An additional number of Americans may also suffer from various other eating disorders that may result in cachexia (i.e., a general physical wasting and malnutrition) or periods of obesity and/or cachexia. The etiology of obesity is largely unknown. The etiology of some eating disorders is psychological in many patients, but for other patients, is poorly understood.
Patients suffering from morbid obesity and/or other eating disorders have very limited treatment options. For instance, some of these patients may undergo surgery to reduce the effective size of the stomach (“stomach stapling”) and to reduce the length of the nutrient-absorbing small intestine. Such highly invasive surgery is associated with both acute and chronic complications, including infection, digestive problems, and deficiency in essential nutrients. In extreme cases, patients may require surgical intervention to a put a feeding tube in place. Patients suffering from eating disorders may suffer long-term complications such as osteoporosis.
As the medical treatments for gastrointestinal motility disorders, sphincteric disorders, and eating disorders have proven ineffective, improvements are still needed for the treatment of these GI disorders providing effective long term results, elimination of the need for lifelong medicine and its attendant side effects, and the relative ease of an endoscopic implantation procedure.